OpenCV: "[ WARN:0] terminating async callback" when attempting to take a picture

Question

I am trying to take a picture from the defualt carmera with python, to do this I am using openCV (import cv2 as cv from python shell). However, when I attempt to disable the camera it closes but with the error [ WARN:0] terminating async callback.

This is code I am trying to run:

import cv2 as cv

camera_port = 0
camera = cv.VideoCapture(camera_port)
return_value, image = camera.read()
cv.imwrite("image.png", image)

camera.release() # Error is here

The code outputs the desired result, it takes and saves an image, but I do not understand why the error message occurs or how to remove it

For whoever is looking for the reason, as Aprajita Verma mentioned, the handle to the webcam is not released which gives the error. I have used the webcam in and outside of a condition in my code for OCR application. When the handle is released, the error does not occur. — Siyahi, Sep 15 '20 at 16:49

score 36 · Accepted Answer · edited Jul 23 '22 at 15:47

36

I had the same warning.

Just modify the line

camera = cv.VideoCapture(camera_port)

to

camera = cv.VideoCapture(camera_port, cv.CAP_DSHOW)

edited Jul 23 '22 at 15:47

Christoph Rackwitz

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answered Jul 01 '19 at 17:12

Karl Djotchuang Tamo

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3

This solution worked but defining port instead `cv2.VideoCapture(0, cv2.CAP_DSHOW)` – isaac weathers Feb 09 '20 at 16:18
4

Using ``cv2.CAP_DSHOW`` removes the warning, but slows down my frame rate from 30fps to 7fps, on Windows. – Janos Mar 01 '20 at 17:13
1

Correct Answer, don't know why, but mentioning the 2nd parameter solves the warning – Engr Syed Rowshan Ali May 29 '20 at 12:36
Goodness! Thank you so much! – Mrigank Pawagi Aug 19 '20 at 16:03
It's worth noting that DirectShow is a legacy API and is currently deprecated – Goigle Apr 02 '21 at 19:51
@Goigle what would you use instead of `CAP_DSHOW`? – Peter Wood May 05 '21 at 20:47
2

@PeterWood The default on windows should be fine (`CAP_MSMF`), assuming your Win10 is up to date. On Win10 20H2 I experience no issues (that warning just means the stream ended). The only people on my team who have issues are on outdated versions of Windows. This makes sense as DSHOW used to be the default and MSMF is slowly replacing DSHOW – Goigle May 06 '21 at 21:12
`destroyAllWindows()` is absolutely _not_ relevant to this question. – Christoph Rackwitz Feb 17 '22 at 12:53

score 7 · Answer 2 · edited Jul 23 '22 at 15:47

7

It's probably showing a warning because you're not releasing the handle to the webcam.

try adding this to the end of the code

camera.release()
cv2.destroyAllWindows()

I hope this helps!

edited Jul 23 '22 at 15:47

Christoph Rackwitz

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answered Jan 04 '19 at 12:20

Aprajita Verma

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On Windows 10 with Python 3.8 and OpenCV 4.2, this does not solve the problem. The warning is also displayed on the OpenCV sample programs, e.g. ``samples/python/video.py``. – Janos Mar 01 '20 at 17:42

score 7 · Answer 3 · edited Jul 23 '22 at 15:51

7

camera = cv.VideoCapture(camera_port, cv.CAP_DSHOW)

cv.destroyAllWindows()

edited Jul 23 '22 at 15:51

Christoph Rackwitz

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answered Mar 12 '19 at 18:22

Sumit kumar

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Using cv2.CAP_DSHOW removes the warning, but slows down my frame rate from 30fps to 7fps, on Windows – Janos Mar 01 '20 at 17:13
bad answer. `destroyAllWindows` has nothing to do with cameras, and there are no windows to destroy. do not just copy a previous answer and repost it. – Christoph Rackwitz Jul 23 '22 at 15:54

score 4 · Answer 4 · edited Feb 08 '19 at 04:36

4

I did this & I don't see that warning there after.(only for Windows OS)

Open cmd and type:

setx OPENCV_VIDEOIO_PRIORITY_MSMF 0

edited Feb 08 '19 at 04:36

Suraj Kumar

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answered Feb 08 '19 at 04:07

Little_being

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Doesn't work on Windows 10 – The Guy Sep 13 '22 at 04:31

score 1 · Answer 5 · edited Jul 23 '22 at 15:51

1

This seems to be a bug in MSMF backend of opencv.

If you are using windows then you can change the backend to DirectShow backend.

So, change VideoCapture like this:

captureDevice = cv.VideoCapture(0, cv.CAP_DSHOW)

edited Jul 23 '22 at 15:51

Christoph Rackwitz

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answered Apr 20 '22 at 05:12

Codemaker2015

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score 0 · Answer 6 · answered Apr 18 '19 at 01:56

0

It works for me as indicated Sumit Kumar


camera_port = 0
#camera = cv2.VideoCapture(camera_port)
camera = cv2.VideoCapture(camera_port,cv2.CAP_DSHOW)
# Check if the webcam is opened correctly
if not camera.isOpened():
    raise IOError("Cannot open webcam")

return_value, image = camera.read()
print("We take a picture of you, check the folder")
cv2.imwrite("image.png", image)

camera.release() # Error is here
cv2.destroyAllWindows()

answered Apr 18 '19 at 01:56

Joe Llerena

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bad answer. `destroyAllWindows` has nothing to do with cameras, and there are no windows to destroy. do not just copy a previous answer and repost it. – Christoph Rackwitz Jul 23 '22 at 15:54

score -1 · Answer 7 · edited Jul 23 '22 at 15:52

-1

first:add cv.destroyAllWindows()
second:the camera permission you have banned,and then check it.

edited Jul 23 '22 at 15:52

Christoph Rackwitz

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answered Jan 07 '19 at 09:35

robot liu

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bad answer. `destroyAllWindows` has nothing to do with cameras, and there are no windows to destroy. do not just copy a previous answer and repost it. – Christoph Rackwitz Jul 23 '22 at 15:53

score -1 · Answer 8 · edited Jul 23 '22 at 15:52

-1

camera = cv.VideoCapture(camera_port, cv.CAP_DSHOW) # Added cv.CAP_DSHOW
return_value, image = camera.read()
cv.imwrite("image.png", image)
camera.release()
cv.destroyAllWindows() # Handles the releasing of the camera accordingly

edited Jul 23 '22 at 15:52

Christoph Rackwitz

11,317
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answered Dec 17 '19 at 03:51

Brady R

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bad answer. `destroyAllWindows` has nothing to do with cameras, and there are no windows to destroy. do not just copy a previous answer and repost it. – Christoph Rackwitz Jul 23 '22 at 15:53
Please read [How do I write a good answer?](https://stackoverflow.com/help/how-to-answer). While this code block may answer the OP's question, this answer would be much more useful if you explain how this code is different from the code in the question, what you've changed, why you've changed it and why that solves the problem without introducing others. – Saeed Zhiany Jul 24 '22 at 06:41

score -1 · Answer 9 · answered Feb 04 '20 at 05:46

-1

hey guys found the solution pip install opencv-contrib-python==3.4.7.28 try like this we have to specifically say the version try lesser version mine was 4.x so I did and no error popped up

answered Feb 04 '20 at 05:46

ARUN AK

1

that only does something because the **older** version doesn't have the new MSMF API, which emits this _warning_ – Christoph Rackwitz Jul 23 '22 at 15:53

OpenCV: "[ WARN:0] terminating async callback" when attempting to take a picture

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